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Thread: Ff dof

  1. #1
    Ricco's Avatar
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    Ff dof

    I've previously read on this site and others that having a full frame camera vs crop sensor changes the depth of field of an image. This doesn't quite make sense to me and I was hoping someone could explain.

    My logic is that I have read that a given lens effectively produces the same image on the sensor regardless of the body. Thus if I have a ef lens on a crop body, I am only getting part of the picture as opposed to having a full frame. This example is often used to explain the "zoom" that you get with a crop camera (and yes, I know it is not actually zoom but instead it is just sampling a smaller part of a bigger picture). This is all good and makes sense. Where I get confused is if it is the same image, then why is the dof affected?

    So - what I am asking is if I had the same lens on a 5d and 60d (for example), with the same camera settings, is the dof the same or different?

    Have I got myself confused somewhere?

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    Snarkbyte's Avatar
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    Re: Ff dof

    Hi Peter,

    The difference is due to the size of the pixel elements on the different sensors... if the FF and crop-sensor bodies had sensors with the pixel elements of exactly the same dimensions, there would be no difference in DoF (assuming everything else, such as focal length, aperture, and range to target are the same). The size of the pixel determines maximum sharpness of the image (assuming a "perfect" lens), so a sensor with a smaller pixel size will show loss-of-sharpness sooner than a sensor with a larger pixel size. In other words, greater pixel density (smaller pixels) means shallower DoF (in theory, anyway), because the limit of maximum sharpness is smaller. The difference is really in the way we define "acceptable sharpness"... if "acceptable sharpness" is determined by the size of the pixel element, then the limits of acceptable sharpness are smaller for the sensor with higher pixel density. In general, FF sensors have larger pixel elements (which is good for low-light capability), and crop-sensor bodies have higher pixel DENSITY (which is good for sharpness). Make sense?

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    Re: Ff dof

    Ricco - The reason the depth of field is reduced is that if you want the "same" picture (i.e. framing) then you have to move in closer to the subject with the full frame camera which reduces the DOF. IMHO, it is really not an issue and not a reason to chooses one format over the other. I'm sure some will disagree.

    John

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    Re: Ff dof

    Just to expand a little further on what John said, if you want to get identical framing with the larger format, you either have to get closer to your subject (using the same lens), thereby decreasing the DoF, or you stand in the same place, and use a longer lens, which also decreases the depth of field.

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    Re: Ff dof

    Quote Originally Posted by PhotomanJohn View Post
    IMHO, it is really not an issue and not a reason to chooses one format over the other. I'm sure some will disagree.
    Just for fun ...

    If the difference between a crop-factor camera and a full-frame camera in terms of DoF is about a stop - then would one not have to stop-down the full-frame camera 1 more stop - thus halving the light hitting the sensor - and thus need TWICE the light for the same shutterspeed? So are we saying you only need 1/2 the light with a crop-factor camera for a given depth of field?

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    Ricco's Avatar
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    Re: Ff dof

    Ummmmm........

    I think I have got myself more confused by asking the question. There are a couple of plausible explanations but I'm not sure which is correct?

    If i start with a ff and crop camera, same lens and same distance to subject - ignoring the effect of pixel density, I am assuming that the pictures on the back of camera are the same (acknowledging one will be cropped).

    Al's explanation makes sense to me in that pixel density has an effect on sharpness - however, that suggests to me that a full frame camera is inherently softer - now this doesn't make sense but I'm willing to go with that if it is the case?

    With regards to moving closer or further from the subject - again, this is all plausible, but my impression when I've been reading is that the dof is inherent with the camera, not necessarily related to positioning of the subject.

    I guess taking that point a little further, with a full frame you need to zoom a little more leading to effectively less reflective light and therefore greater dof required for a given shot.

    So Colin, how does this sit with previous comments that you need a greater shutter speed for a crop camera to avoid shake? If the picture on the element is the same, how is this affected?


    Thanks

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    Re: Ff dof

    `
    Quote Originally Posted by Ricco View Post
    So Colin, how does this sit with previous comments that you need a greater shutter speed for a crop camera to avoid shake? If the picture on the element is the same, how is this affected?
    Hi Peter,

    Sorry, but I am SO "staying out of this one" (Getting the marshmellows ready to toast though!)

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    Re: Ff dof

    Quote Originally Posted by Snarkbyte View Post
    Hi Peter,

    The difference is due to the size of the pixel elements on the different sensors... if the FF and crop-sensor bodies had sensors with the pixel elements of exactly the same dimensions, there would be no difference in DoF (assuming everything else, such as focal length, aperture, and range to target are the same). The size of the pixel determines maximum sharpness of the image (assuming a "perfect" lens), so a sensor with a smaller pixel size will show loss-of-sharpness sooner than a sensor with a larger pixel size. In other words, greater pixel density (smaller pixels) means shallower DoF (in theory, anyway), because the limit of maximum sharpness is smaller. The difference is really in the way we define "acceptable sharpness"... if "acceptable sharpness" is determined by the size of the pixel element, then the limits of acceptable sharpness are smaller for the sensor with higher pixel density. In general, FF sensors have larger pixel elements (which is good for low-light capability), and crop-sensor bodies have higher pixel DENSITY (which is good for sharpness). Make sense?
    So you're saying a 5DMk II has more DoF than a 5D or if I dug out one of my film bodies I'd get more DoF by using a 100iso film than I would with a 400iso film?


    ....and Colin, make room I'm coming to join you in your retreat.

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    Re: Ff dof

    Quote Originally Posted by black pearl View Post
    ....and Colin, make room I'm coming to join you in your retreat.
    Could be a fireworks display later on too

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    Moderator Dave Humphries's Avatar
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    Re: Ff dof

    Colin, Robin, You two need your heads banging together

    Someone is gonna have to clear this mess up

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    Moderator Dave Humphries's Avatar
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    Re: Ff dof

    Hi Peter,

    I think where you were going 'wrong' was when talking about the image from a given lens (i.e. same focal length) on FF vs crop bodies with statements like (para-phrasing) 'the image is the same, except one is cropped' - the italics bit really means the image isn't the same in my opinion.

    You must make the image the same in order to compare DoF.

    So you either you zoom in more on the FF camera, as Kathy said, which changes the DoF.

    Or you crop down and enlarge the FF's wider angle view to give the same image angle of view as the crop body, which will make the areas that seemed sharp before less so, effectively changing the DoF, if you see what I mean.

    Cheers,

  12. #12
    Moderator Dave Humphries's Avatar
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    Re: Ff dof

    Hi John, you knew this was coming, right?

    Quote Originally Posted by PhotomanJohn View Post
    IMHO, it is really not an issue and not a reason to chooses one format over the other. I'm sure some will disagree.
    I agree that within the confines of FF and APS-C, or maybe even going out to m4/3, DoF isn't really an issue to influence format choice.

    Although: FF f/4 = APS-C f/2.8 = m4/3 f/2 for DoF, meaning thin (artistic) DoF attainable with FF at f/2 is just not available to m4/3 users with off the shelf lenses. (but how often is that thin a DoF needed in reality?)

    However, once you go to say, Nikon 1 (c/f 2.7) and beyond to even smaller sensor cameras, the lack of artistic control of DoF is going to become a real PITA, especially at longer focal lengths where the lens can't even manage the advertised wide aperture of say, "f/2".

    On the other hand; for macro all that extra DoF is a massive help.

    Cheers,
    Last edited by Dave Humphries; 18th February 2012 at 02:32 PM.

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    Re: Ff dof

    I'll take a stab at this with the risk of fireworks being shot my way.


    What effects depth of field?.......................2 of the main things: focal length---distance to the subject

    2 cameras side by side with the same lens and f/stop. one is a 5d FF and the other is a 1.6 crop 60d

    If you take a shot with both, i think the result would be the 60d would have a shallower depth of field because it has 1.6X the effective focal length.

    If you frame the shot exactly the same, in camera. The FF 5d would have a shallower depth of field because you would have to get much closer to the subject (which has a greater effect on dof than the f/stop will)


    OK bring on the fireworks.

  14. #14
    Moderator Dave Humphries's Avatar
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    Re: Ff dof

    Quote Originally Posted by Steve S View Post
    If you frame the shot exactly the same, in camera. The FF 5d would have a shallower depth of field because you would have to get much closer to the subject (which has a greater effect on dof than the f/stop will)
    As above, you must do this, or you are comparing apples and oranges (IMHO).

    I think the less confusing way to think of it is to use a longer focal length, rather than move shooting position, because moving changes perspective, which is far more noticeable than DoF (now we're comparing apples and pears).

    By changing sensor format, both the apparent focal length (i.e. real x crop factor) AND the DoF for a given aperture are affected (f/stop x crop factor).

    Cheers,
    Last edited by Dave Humphries; 18th February 2012 at 02:54 PM.

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    Re: Ff dof

    Quote Originally Posted by Snarkbyte View Post
    Hi Peter,

    The difference is due to the size of the pixel elements on the different sensors... if the FF and crop-sensor bodies had sensors with the pixel elements of exactly the same dimensions, there would be no difference in DoF (assuming everything else, such as focal length, aperture, and range to target are the same). The size of the pixel determines maximum sharpness of the image (assuming a "perfect" lens), so a sensor with a smaller pixel size will show loss-of-sharpness sooner than a sensor with a larger pixel size.
    In other words, greater pixel density (smaller pixels) means shallower DoF (in theory, anyway), because the limit of maximum sharpness is smaller. The difference is really in the way we define "acceptable sharpness"... if "acceptable sharpness" is determined by the size of the pixel element, then the limits of acceptable sharpness are smaller for the sensor with higher pixel density. In general, FF sensors have larger pixel elements (which is good for low-light capability), and crop-sensor bodies have higher pixel DENSITY (which is good for sharpness). Make sense?
    Don't really understand the below. To me it defies common sense.
    Assuming a infinitely small pixel size, that would mean the more closely examine the sharpness of an edge, there would be NO difference in the sharpness. With a relatively large pixel size, there HAS to be a point (as you examine more closely) that you get down to level where each individual pixel can be seen - meaning that the LARGER pixel would show a loss of sharpness before the smaller pixel. Or at least that's how I understand it. And that's based on the assumption that each pixel operates perfectly.

    Memory says I read an article ages ago about DoF (so I may be wrong here), and please note, I'm talking here about DoF not sharpness..
    Take a picture using a 50mm lens at f4, take the same picture of the same subject with a 500mm at f4.
    Now, crop into the 50mm image so the final image is the same as the 500mm image. The DoF should be fairly close (if not the same). The perspective also stays the same (remember you haven't moved from you position for both shots).
    The DoF is a zone around the actual focal plane that SEEMS to be sharp. So, if you have a wide angle shot, more of the content seems to be in focus than if you could actually see the level of detail required. If you take a telephoto shot, you have ALSO zoomed in on the zone around the actual focal plane, hence you can see the out of focus sections more clearly.

    Make sense?

    Graham

  16. #16
    Black Pearl's Avatar
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    Re: Ff dof

    Yep.

  17. #17

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    Re: Ff dof

    This is an old question, which I remember well from the days of film. Briefly, for a given field of view, a smaller image format gives a greater DoF at any given f-number. This has the useful effect for casual users that most pictures are more-or-less sharp. Using a larger format gives a greater ability to use selective focus, to throw backgrounds out-of-focus. Not to mention a better IQ overall...

    You can test this quite easily - choose an image with a deep field, and fill the frame using different format cameras. Then it's time to pixel-peep

    HTH

    Peter

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    Re: Ff dof

    I should follow Colin's lead but I think I will make a couple of more comments. When you talk about DOF forget about the "effective" or "equivalent" focal length based on one format size to another. Doing so tends to make the subject more complicated and leads to incorrect conclusions as noted above.

    In regards to the 50mm versus 500mm example, the 50mm lens will always have more depth of field and as the subject distance approaches the hyperfocal distance of the shorter lens there will be no comparison between the two DOFs.

    I understand why individuals new to photography get confused because there is so much contradictory information on the subject. Good luck Peter in sorting through it all.

    John

  19. #19
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    Re: Ff dof

    You got a lot of answers but I don't think it has been explained plainly. The DOF is identical on the crop camera and the FF camera using the same f/stop and distance to subject if both images are enlarged by the same percent. However, that means the two images will not be the same size because the crop image starts out smaller. If both images were enlarged so they were the same size then the DOF would be different. Why? With the greater enlargement of the crop image the circle of confusion is larger and the image will not appear as sharp. DOF is affected by f/stop, camera-to-subject distance, enlargement size and viewing distance.

  20. #20
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    Two edged sword

    The full frame camera is has both a wider and a more narrow DOF as compared with a crop camera...

    How can this be?

    If a subject was shot with both a full frame camera and a crop camera from the same distance using the same focal length lens and at the same f/stop: the image shot with the full frame camera would have the greater DOF. That is because the full framer uses a larger circle of confusion than the crop camera. All calculations from DOF Master ( http://www.dofmaster.com/dofjs.html )

    If you focused at 10 feet with a 50mm lens, shooting with f/4 on a 1.6x crop camera and the same on a full frame camera; the DOF of the cropper would be 1.84 feet while the DOF of the full framer would be 2.94 feet.

    See! I told you that the full frame camera has a greater DOF than a crop camera...

    Circle of confusion for a 1.6x crop camera = .019mm
    Circle of confusion for a full frame camera = .03mm

    However! If you were shooting at f/4 with both cameras and wanted the exact same framing at 10 feet with a crop camera as you get with a full frame camera, you would shoot with a 50mm lens on the cropper giving you a DOF of 1.84 feet AND you would shoot with an 80mm lens on a full frame camera giving you a DOF of 1.12 feet.

    See, I told you that the full frame camera has a more narrow DOF than the crop camera, or did I?

    AND... if you were shooting with 50mm lenses on both cameras and wanted to frame the image the same, you would need to move closer to the subject and therefore the full frame camera would have a more narrow DOF than the cropper...

    That is what I've been telling you all along, or did I?

    Two out of three for the full framer having the more narrow DOF.
    Last edited by rpcrowe; 19th February 2012 at 12:11 AM.

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